I suppose we should be thankful that, for a change, a great big summer blockbuster has arrived that isn’t chockablock with comic-book superheroes or buzzcut, maxi-muscled marauders. “Tomorrowland” is a rather sweet excursion into speculative sci-fi, and, wonder of wonders, it doesn’t even seemed primed for a sequel. But this movie about the thrill of the visionary is, alas, mostly earthbound.

Directed and co-written (with Damon Lindelof and Jeff Jensen) by Brad Bird, who made the marvelous animated features “The Iron Giant” and “The Incredibles,” the film centers on Casey Newton (Britt Robertson), a precocious Texas teen whose dad (Tim McGraw) was once a NASA engineer at a rocket launch site. Now, to her horror, the site is being demolished.

Casey becomes the recipient of a mysterious pin with a big “T” emblem, which briefly enables her to enter what appears to be a model city of the future, complete with model people. Back home in the present, she is soon aided by a benevolent pre-teen bot (Raffey Cassidy) and pursued by bot meanies, ending up paired with Frank Walker (George Clooney), an ornery hermit scientist who may hold the key to a return trip to Tomorrowland. This is assuming it exists at all and is not a wish-fulfillment fantasy.

Considering that Bird comes out of animation, the visuals in this movie are for the most part surprisingly prosaic (like something out of, maybe, “Spy Kids.”) A vision of the resplendent (or not-so-resplendent) future ought to carry more oomph than this. Still, there are compensations: Robertson and Cassidy are spunky and Clooney is amusingly grizzled, and there’s a marvelous set piece where the middle of the Eiffel Tower suddenly turns into a great big rocket shooting into the night sky. The film’s eco-friendly concerns and general tone of messianic optimism are welcome. “Tomorrowland” is really a civics lesson disguised as a summer escape flick. Grade: B- (Rated PG for sequences of sci-fi action violence and peril, thematic elements, and language.)